1. Field of the Invention
The present invention relates to an improved process for preparing alkoxysilanes from halosilanes and alcohols.
2. Discussion of the Background
Alkoxysilanes, e.g., of the formula I below, are important industrial intermediates or end products in organosilane chemistry. They are used, inter alia, as coupling agents in composite materials, for example in the coatings and glass fiber industry, in the foundry and in the manufacture of adhesives, and for the manufacture of elastomers. Individual examples which can be mentioned are silanized glass fibers, polymer systems or silicone systems reinforced by fine materials, e.g., permanently flexible sealants, silica-filled rubber articles, e.g. tires, the modification of hydroxy-functional surfaces, the silane polycondensation to give polyorganosiloxanes and preservatives for buildings and other structures.
The alkoxysilanes are prepared by reacting halosilanes, which can contain one or more halogen atoms, with an alcohol, for example an alkanol or an alkoxyalkanol, in accordance with the general equation below: EQU R.sup.1.sub.a R.sup.2.sub.b R.sup.3.sub.c SiX.sub.4-b-c +(4-a-b-c)R.sup.4 OH.fwdarw.R.sup.1.sub.a R.sup.2.sub.b R.sup.3.sub.c Si(OR.sup.4).sub.4-a-b-c +(4-a-b-c)HX
In the formulae, R.sup.1, R.sup.2, R.sup.3, R.sup.4 and X, a, b and c, have the meaning specified below. As a by-product, the stoichiometric amount of hydrogen halide HX is therefore always formed.
The halosilanes are reacted with the alcohol either batchwise or continuously, the hydrogen halide formed being converted into the gas phase or remaining bound in the liquid phase. Customary techniques for removing the hydrogen halide by conversion into the gas phase are stripping, distillation (also reactive distillation). Processes of this type are described, inter alia, in DE 20 61 189 and U.S. Pat. No. 4,298,753 (continuous reactive distillation); DE 28 00 017, DE-A 38 01 618, DE-A 24 09 731 and DE-A 27 44 726 (distillation of the crude product to remove hydrogen chloride); DE-A 32 36 628 (connecting stirred tank and column in series); DE 862 895 and DD 31 751 (stripping of hydrogen chloride by inert gas). Considerable disadvantages of these processes for removing hydrogen halide by conversion into the gas phase are the secondary reactions of the hydrogen halide with the alkoxysilanes produced to form siloxanes according to the reaction equation: ##STR1## and the limitation of the degree of conversion of the halosilanes to alkoxysilanes by the thermodynamic equilibrium owing to the presence of hydrogen halide in the reaction mixture. This applies particularly if more than one halogen atom is to be replaced by an alkoxy radical. In this case, non-alkoxylated, mono- and polyalkoxylated silanes are present simultaneously when trihalosilanes are reacted with alcohol, the equilibrium constants decrease from the first to the third stage. That is to say that the third alkoxy group especially is difficult to introduce or that introduced third alkoxy groups readily react with hydrogen halide to reform a halosilane structure.
A sought-after alkoxysilane is (3-chloropropyl)methyldimethoxysilane (CPMDMO), which is formed from (3-chloropropyl)methyldichlorosilane and methanol. In this case also, the reaction product of the first stage, the monoalkoxychlorosilane, is favored over the dialkoxysilane. This is shown by comparing the equilibrium constants, that for the first stage at room temperature being considerably greater than 1, but that for the second stage being only 0.4. In a known process the reaction is carried out batchwise in a stirred-tank reactor, and the resulting hydrogen halide is removed from the reaction mixture by distillation. The yield is about 60%. It can be increased to 65% by adding methanolic sodium methoxide solution, but with the disadvantage that sodium chloride is produced and the risk that a 3-methoxypropyl group will result from the 3-chloropropyl group in a Williamson synthesis.
The disadvantages of the processes in which the hydrogen halide formed is converted into the gas phase are particularly serious because the transfer of the hydrogen halide from the liquid phase to the gas phase proceeds relatively slowly and thus the mass transport of the hydrogen halide has a critical effect on the entire process. The equilibrium is therefore shifted only comparatively slowly in favor of the wanted alkoxylated products, and the hydrogen halide promotes the formation of siloxanes, in accordance with the above equation. The hydrogen halide is particularly slowly converted to the gas phase in reactions with methanol, since it is readily soluble in methanol. Owing to the slow transfer of the hydrogen halide to the gas phase, the yields of alkoxysilane are decreased.
Another technique for removing the resulting hydrogen halide from the reaction mixture, which has already been motioned in connection with the synthesis of(3-chloropropyl)-methyldimethoxysilane, is binding the hydrogen halide in the liquid phase by reaction with basic substances. However, if acid-binding agents are used, e.g. ammonia or (tertiary) amines (DE 913 769, which describes the removal of hydrogen chloride by ammonia), or if alkoxides are used, stoichiometric amounts of salt are produced which need to be separated off from the product.
The use of solvents in the removal of hydrogen halide is likewise described. In this case the solvents serve to decrease the viscosity of the reaction mixture, which is always a single phase (DE-A 20 61 189), or to depress the boiling point of the reaction mixture, which is always a single phase (DE-A 28 00 017, DE-A 32 36 628). In contrast, the use of solvents in a two-phase reaction mixture has not yet been described.
In contrast, it is known that it is advantageous to select a reaction temperature &lt;100.degree. C. when the halosilane is reacted with the alcohol, to avoid unwanted side reactions to form siloxanes (DE-A 24 03 731). This requires hydrogen halide to be removed at the lowest possible temperature, although higher temperatures would be more favorable for this removal.
For the preparation of alkoxysilanes, the use of various reactors is described, that is to say stirred tanks (e.g. GB 674 137), tubular reactors--(DE-A 20 33 373), stirred tanks equipped with a column (e.g. --DE-A 32 36 328) and reaction-distillation columns (e.g., U.S. Pat. No. 4,298,753).